Hitachi Review Vol. 63 (2014), No. 9 602 Featured Articles Light Water Reactor System Designed to Minimize Environmental Burden of Radioactive Waste Tetsushi Hino, Dr. Sci. OVERVIEW: One of the problems with nuclear power generation is the Masaya Ohtsuka, Dr. Eng. accumulation in radioactive waste of the long-lived TRUs generated as Kumiaki Moriya byproducts of the fission of uranium fuel. Hitachi is developing a nuclear reactor that can burn TRU fuel and is based on a BWR design that is Masayoshi Matsuura already in use in commercial reactors. Achieving the efficient fission of TRUs requires that the spectrum of neutron energies in the nuclear reactor be modified to promote nuclear reactions by these elements. By taking advantage of one of the features of BWRs, namely that their neutron energy spectrum is more easily controlled than that of other reactor types, the new reactor combines effective use of resources with a reduction in the load on the environment by using TRUs as fuel that can be repeatedly recycled to burn these elements up. This article describes the concept behind the INTRODUCTION RBWR along with the progress of its development, NUCLEAR power generation has an important role to as well as its specifications and features. play in both energy security and reducing emissions of carbon dioxide. One of its problems, however, is the accumulation in radioactive waste from the long- RBWR CONCEPT lived transuranium elements (TRUs) generated as Plutonium Breeding Reactor byproducts of the fission of uranium fuel. The TRUs The original concept on which the RBWR is based include many different isotopes with half-lives ranging was the plutonium generation boiling water reactor from hundreds to tens of thousands of years or more. (PGBR) proposed by Takeda and others from Hitachi As a result, the radiotoxicity of TRU-containing in 1988(2). The PGBR produces fissile plutonium (Puf), radioactive waste (a measure of the intensity of a TRU fuel for nuclear power generation containing radiation in terms of the combined effects on people plutonium-239 (239Pu) and plutonium-241 (241Pu), of all radioactive isotopes in the waste) takes around from uranium-238 (238U), the non-fissile isotope that 100,000 years to fall to a level equivalent to that of makes up more than 99% of natural uranium (U). naturally occurring uranium ore. If, on the other hand, Puf breeding means that more Puf is created during the TRUs could be burned up to eliminate them from the burning of the fuel than is provided in the initial the radioactive waste, this time could be reduced to a fuel as a startup neutron source. As it was generally few hundred years(1). assumed at the time that this could only be achieved Work is progressing on the research and using sodium-cooled fast reactors, the PGBR’s ability development of nuclear reactors capable not only of to breed plutonium in a light water reactor made it a eliminating TRUs from waste, but also of reducing ground-breaking proposal. the consumption of uranium by burning TRUs as fuel. To achieve the breeding of plutonium, it is Sodium-cooled fast reactors, which use sodium (Na) necessary to promote the absorption of neutrons by to cool the fuel, are one example. 238U to transmute it into 239Pu. For it to be used in a Hitachi, meanwhile, is working on the development power reactor, it also needs to be able to sustain a of its resource-renewable boiling water reactor fission chain reaction in the nuclear reactor. That is, (RBWR) based on a boiling water reactor (BWR) a higher number of neutrons than in a conventional design that is already in use in commercial reactors. BWR is required to maintain the simultaneous - 75 - 603 Light Water Reactor System Designed to Minimize Environmental Burden of Radioactive Waste transmutation and fission chain reactions. Typically, it automatically acts to reduce that output. The main the higher the energy of the neutrons that trigger mechanisms for inherent safety in a conventional fission, the more it encourages neutron absorption by BWR are the effect whereby fission is suppressed 238U and the higher, in relative terms, the number of by the greater absorption of neutrons as the fuel neutrons emitted by the fission reaction. Accordingly, temperature increases (Doppler effect), and the effect breeding plutonium requires that the mean neutron whereby higher temperatures promote more coolant energy in the reactor be raised. boiling, which raises the proportional volume of steam In a BWR, the heat from the fuel rods causes (void fraction), reduces the moderation of neutrons, the water (coolant) that flows through the core of a and thereby also suppresses fission. nuclear reactor to boil, thereby removing heat from This latter effect is a consequence of the fact that, the fuel rods. Meanwhile, the coolant also acts to in a conventional BWR, fission is primarily triggered moderate the high energy of the neutrons generated by low energy neutrons (around 0.1 eV). This is by the fission reaction (reduce it down to a low level) because, at low energies, the fission of fissile 235U through repeated collisions between neutrons and and 239, 241Pu tends to become less likely as the neutron the hydrogen nuclei in the coolant. To minimize this energy increases. An indicator that represents this effect, the PGBR reduces the proportional volume relationship between void fraction and likelihood of of coolant in the reactor core by having narrower fission is called the void reactivity coefficient. The coolant channels between fuel rods. Calculations void reactivity coefficient has a negative value in have also demonstrated that the PGBR can achieve a situations where an increase in void fraction makes high neutron energy and enable Pu breeding despite fission less likely, as in a conventional BWR. being a light water reactor by taking advantage of a This relationship reverses at high energies of characteristic of BWRs whereby the boiling of coolant 100 keV or more, where the higher the neutron energy to form steam reduces the density of hydrogen nuclei. the more likely fission is to occur. Accordingly, when the proportion of fission reactions triggered by high- Intrinsic Safety energy neutrons becomes large, as in the PGBR which Nuclear reactors need to exhibit inherent safety, which is intended for plutonium breeding, the effect whereby means that, when the output of a reactor increases due a higher void ratio automatically acts to reduce reactor to some external factor, the nature of its design means output becomes attenuated. Although the Doppler Turbine building Conventional BWR fuel 155 mm Reactor building 4 m Fuel rod Coolant RBWR fuel Core TRU zone (including Puf) 2.4 m 199 mm Reactor Power plant pressure vessel 194 mm BWR: boiling water reactor RBWR: resource-renewable BWR TRU: transuranium element Puf: fissile plutonium Fig. 1—RBWR Concept. Apart from the core, the RBWR is largely the same as conventional BWR systems. To achieve effective burning of TRUs, the gaps between fuel rods through which the coolant flows are narrower on the RBWR. Also, the TRUs are contained in two separate zones to ensure inherent safety. - 76 - Hitachi Review Vol. 63 (2014), No. 9 604 depleted uranium is a plentiful resource that can Rise in thermal output provide the basis of a long-term energy supply. Multi-recycling requires that the isotopic More coolant boiling composition of TRUs in the fuel be kept the same TRU zone (including before and after burning them in the reactor. If the Puf) proportion of fissile isotopes after use is lower, then Low Fewer fission High the amount of these isotopes will diminish each time neutron reactions neutron leakage leakage the fuel is recycled, ultimately resulting in loss of Lower criticality in the reactor. There is also the risk of thermal output Depleted uranium compromising reactor design and operation criteria, zone (no Puf) such as a change in the fuel composition causing Low thermal output High thermal output the void reactivity coefficient to become positive. In addition to having narrower coolant channels, the Fig. 2—Inherent Safety of RBWR. RBWR-AC is able to satisfy a variety of criteria as it A rise in thermal output leads to more coolant boiling and runs through repeated operation cycles by modifying greater neutron leakage, thereby suppressing fission. the coolant flow rate to adjust the neutron energy spectrum during operation and maintain a constant isotopic composition of TRUs in the fuel before and effect still works automatically to reduce output, the after use. PGBR has a positive void reactivity coefficient. Takeda and his team continued their work, and TRU Burner in 1995 proposed the concept of an RBWR actinide One of the advantages of nuclear power generation recycler (RBWR-AC) that combined plutonium is that it has a higher energy density than thermal breeding with a negative void reactivity coefficient(3). and other forms of power generation, meaning that The name of the RBWR derives from its ability to it requires less fuel to produce the same amount of recycle not only plutonium but also the other TRUs electric power. On the other hand, when TRUs are as fuel. A feature of the RBWR-AC is that Puf is used to fuel a nuclear reactor, the percentage of the added as a startup neutron source to the fuel in two initial fuel load burned up in each operation cycle is separate zones (two-zoned core) (see Fig. 1). Because only in the single digits or low teens. Accordingly, the increase in void fraction as the reactor output rises the fuel needs to be recycled many times to burn up a acts to reduce the probability that neutrons will collide large amount of TRUs.